Two-stage lateral bulk acoustic wave filter

ABSTRACT

Acoustic wave filter devices are disclosed. A device includes a layer providing or on a topmost layer of an acoustic reflector. The intermediary layer has a first region and a second region. The first region has a first layer thickness and the second region has a second layer thickness different from the first layer thickness. The device includes a first multilayer stack on the first region and a second multilayer stack on the second region of the intermediary layer. Each of the first and the second stacks includes a piezoelectric layer on a counter electrode that is located on the respective region, an input and an output electrode. Application of a radio frequency voltage between the input electrode and the counter electrode layer of the first stack creates acoustic resonance modes in the piezoelectric layer between the input and output electrodes of the first and the second stack.

BACKGROUND Technical Field

This specification relates to thin film radio-frequency acoustic wavefilters.

Background

Radio-frequency (“RF”) components, such as resonators and filters, basedon microacoustic and thin-film technology are widely used in radioapplications such as: mobile phones, wireless networks, satellitepositioning, etc. Their advantages over their lumped-element, ceramic,and electromagnetic counterparts include small size and mass-productioncapability.

SUMMARY

This specification describes technologies for band pass Lateral BulkAcoustic Wave (“LBAW”) filters. More particularly, the presentdisclosure provides techniques to suppress sidebands in LBAW filters andimprove band pass filter characteristic of LBAW filters.

LBAWs can be used as band pass filters. The band pass filter may includeone or more undesired (or parasitic) sidebands. Implementations of thepresent disclosure provide techniques to suppress the undesiredsidebands by cascading two or more LBAWs.

LBAW filters are formed from a piezoelectric layer sandwiched betweentwo pairs of electrodes. One electrode from each pair is located on thetop surface of the piezoelectric layer, and forms an input or an outputof the LBAW. The input and output electrodes are separated by a gap.Each pair also has a counter electrode located on the bottom surface ofthe piezoelectric layer. By applying an alternating voltage across thepiezoelectric layer at the input resonator, a mechanical resonance isformed in the piezoelectric layer below the input electrode. Thepiezoelectric layer thickness and the gap between electrodes can bedesigned such that this mechanical resonance is coupled across the gapto the output resonator. The frequency range at which such couplingoccurs determines the achievable bandwidth (or width of passband) forthe LBAW filter.

In general, one innovative aspect of the subject matter described inthis specification can be embodied in an acoustic wave filter devicethat includes an acoustic reflector, an intermediary layer providing oron a topmost layer of the acoustic reflector, a first multilayer stackon the acoustic reflector, and a second multilayer stack on the acousticreflector and adjacent to the first multilayer stack. The intermediarylayer has a first region and a second region, the first region having afirst layer thickness and the second region having a second layerthickness different from the first layer thickness.

The first multilayer stack includes a first counter electrode on thefirst region of the intermediary layer, and a first piezoelectric layeron the first counter electrode, and a first input electrode and a firstoutput electrode on the first piezoelectric layer. The first inputelectrode and the first output electrode each has a first electrodethickness and extends substantially in parallel and separated by a firstgap. The second multilayer stack includes a second counter electrode onthe second region of the intermediary layer, a second piezoelectriclayer on the second counter electrode, and a second input electrode anda second output electrode on the second piezoelectric layer. The secondinput electrode and the second output electrode each has a secondelectrode thickness and extends substantially in parallel and separatedby a second gap.

The first output electrode is electrically connected to the second inputelectrode. The first multilayer stack and second multilayer stack areconfigured such that application of a radio frequency voltage betweenthe first input electrode and the first counter electrode layer createsacoustic modes in the piezoelectric layer between the first input andoutput electrodes and between the second input and output electrodes. Insome examples, application of a radio frequency voltage between thefirst input electrode and the first counter electrode layer createsacoustic thickness-extensional resonance modes in the piezoelectriclayer between the first input and output electrodes and between thesecond input and output electrodes

The foregoing and other embodiments can each optionally include one ormore of the following features, alone or in combination.

The second layer thickness can be greater than the first layerthickness. For example, the second layer thickness can be 1% to 10%greater than the first layer thickness.

The radio frequency voltage between the first input electrode and thefirst counter electrode layer can create second-order acousticthickness-shear (TS2) resonance mode in the piezoelectric layer betweenthe first input and output electrodes and between the second input andoutput electrodes. The second layer thickness can be different than thefirst layer thickness so that a first resonance frequency of TS2resonance mode created between the first input and output electrodes ofthe first multilayer stack differs from a second resonance frequency ofTS2 resonance mode created between the second input and outputelectrodes of the second multilayer stack. For example, the firstresonance frequency can differ by 1% to 8% from the second resonancefrequency. The first resonance frequency can differ from the secondresonance frequency by at least 50 MHz.

In some embodiments, the second electrode thickness is greater than thefirst electrode thickness. In some embodiments, the second electrodethickness is less than the first electrode thickness. For example, thesecond electrode thickness can be 1% to 10% thinner than the firstelectrode thickness.

Each of the first input and output electrodes can have multiple firstextensions. The multiple first extensions of the first input electrodecan be interdigitated with the multiple first extensions of the firstoutput electrode. Each of the second input and output electrodes canhave with multiple second extensions. The multiple second extensions ofthe second input electrode can be interdigitated with the multiplesecond extensions of the second output electrode. In some examples, eachof the first input and output electrodes is a comb structure having themultiple first extensions and each of the second input and outputelectrodes is a comb structure having the multiple second extensions.

The multiple first extensions can be thicker than the multiple secondextensions. For example, the second extension thickness can be 5% to 20%less than the first extension thickness. The second extension thicknesscan be greater than the first extension thickness so that a firstresonance frequency of thickness-extensional (TE1) resonance modecreated between the first input and output electrodes of the firstmultilayer stack is within 1% from a second resonance frequency of TE1resonance mode created between the second input and output electrodes ofthe second multilayer stack.

The multiple second extensions can be substantially parallel to themultiple first extensions. The multiple second extensions can form anangle greater than zero with respect to the multiple first extensions.

The acoustic reflector can be a Bragg mirror.

The first output electrode and the second input electrode can be partsof a common electrode shared between the first multilayer stack and thesecond multilayer stack.

The first output electrode can be connected to the second inputelectrode through a conductive connector. In some examples, theconductive connector has a length extending parallel to long axis of theextensions of at least one of the first and the second multilayer stacksthat is greater than a width of the extensions measured perpendicular tothe long axis of the extensions.

The first output electrode can be substantially parallel to the secondinput electrode.

The subject matter described in this specification can be implemented inparticular embodiments so as to realize one or more of the followingadvantages. Band pass filters described herein can improve the band passresponse of LBAW filters, e.g., by suppressing parasitic sidebands. Thesuppression can be achieved at particular frequencies or over a range offrequencies. In addition, LBAW filters described herein can be simplerto fabricate as compared to conventional acoustic filters because anLBAW uses only a single piezoelectric layer as compared to twovertically stacked bulk acoustic wave (BAW) coupled resonator filters.The LBAW filters can also operate at higher frequencies as compared tosurface acoustic wave (SAW) filters because the LBAW filter operation isdetermined more by piezoelectric layer thickness than interdigitaltransducer (IDT) electrode dimensions. In some embodiments, LBAW filterscan also achieve a wider bandwidth than BAW filters. LBAW filters canperform as filters with a single lithographic patterning step ascompared to close to 10 in BAW and can operate without reflectors neededin SAW, and thus in smaller size.

The details of one or more embodiments of the subject matter of thisspecification are set forth in the accompanying drawings and thedescription below. Other features, aspects, and advantages of thesubject matter will become apparent from the description, the drawings,and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic perspective view of a solidly-mounted LBAWfilter.

FIG. 1B is a schematic perspective view of a self-supported LBAW filter.

FIG. 1C is a schematic planar view of an IDT electrode structure.

FIGS. 2A-B are schematic diagrams of two types of propagating plate wavemodes in LBAW piezo layer.

FIG. 3 is a dispersion curves for an exemplary LBAW.

FIG. 4A is a schematic diagram of two resonant modes in an LBAW.

FIG. 4B is an illustrative transmission response of an LBAW as afunction of frequency.

FIG. 5 is an experimental transmission curve of an LBAW as a function offrequency. FIGS.

FIG. 6A-6D illustrate block diagrams of example filter devices thatincludes two example LBAW filters connected in series.

FIG. 6E illustrates part of a cross sectional view of the filter deviceillustrated in FIG. 6A.

FIG. 7A depicts transmission curves of two individual example LBAWs.

FIG. 7B depicts dispersion curves of the two individual example LBAWs ofFIG. 7A.

FIG. 7C depicts transmission curve of an example filter device withcascaded LBAWs.

Like reference numbers and designations in the various drawings indicatelike elements.

DETAILED DESCRIPTION

FIGS. 1A, 1C show an example of an LBAW filter (or resonator) 100 withinput 150 and output 170 electrodes that have an interdigitated geometry(also called “interdigital transducer” or “IDT” LBAW). LBAW filter 100includes a piezoelectric (“piezo”) layer 110, having a thickness d, anIDT electrode structure 102 located on the top surface of the piezolayer, and a bottom counter electrode 120 located on the bottom surfaceof the piezo layer. IDT electrode structure (“IDT”) 102 includes twocomb-shaped electrodes, 150 and 170, of conductive material, e.g., metalor polysilicon. IDT electrodes 150 and 170 have parallel extensions 150a and 170 a, respectively, that provide the “tines” or “teeth” or“fingers” of the “comb.” Electrode 150 and counter electrode 120 form aninput resonator with piezo layer 110. Electrode 170 and counterelectrode 120 form an output resonator with piezo layer 110.

Acoustic vibrations are created in piezo layer 110 by applying anoscillating (or alternating) input voltage across IDT electrode 150 andbottom counter electrode 120 at an input port 160. The applied voltageis transformed into a mechanical (e.g., acoustic) vibration via thepiezoelectric effect. Under resonance conditions (e.g., with certainacoustic resonance modes, as detailed further below), this vibration cancreate a standing wave under input electrode 150 and an evanescent wave(with exponentially decaying amplitude) in the gap region 190. Withappropriate selection of vibration frequencies and gap width G, thestanding wave can be coupled mechanically across gap 190 from the piezoregion under electrode 150 to piezo region under electrode 170 by theevanescent wave and create a similar standing wave in piezo layer 110under electrode 170. The standing wave under electrode 170 results in anoutput signal voltage with the same frequency at an output port 180 viathe reverse piezoelectric effect. The frequency range at which thiscoupling occurs in mechanical resonance with strong piezoelectriccoupling forms the passband (or bandwidth) of LBAW filter 100. In someexample, the frequency range is between 1.8 and 1.95 GHz. As discussedfurther below, the thicknesses and geometries, and spacing of thevarious layers of LBAW 100 can be tuned to change the RF response andpassband of the filter. Throughout this disclosure, width and length aremeasured along axes parallel to the piezoelectric layer and thickness ismeasured along the axis perpendicular to the piezoelectric layer.

A reflecting structure 130 can serve to isolate the vibration in piezolayer 110 from an underlying substrate 140 and to prevent acousticleakage. The reflecting structure can be a stack of thin layers, forexample, a Bragg reflector composed of alternating high and low acousticimpedance (“Z_(ac)”) material layers. The thickness of these layers canbe designed such that the frequencies with and near the passband of LBAWfilter are reflected back into piezo layer 110 and all other frequenciespass through the mirror.

In some embodiments, LBAW 100 does not directly overlie substrate 140(as shown in FIG. 1A), but is self-supported, as shown in FIG. 1B. Insuch an arrangement, substrate 140 and mirror 130 are replaced by an airgap, with portions of piezo that extend laterally past the region inwhich LBAW 100 is fabricated being supported by substrate 140.

In some embodiments, as shown in FIG. 1C, extensions 150 a and 170 a arerectangular and have a width W, length L, and are spaced by gap width G.Each electrode 150 and 170 has one or more extensions 150 a and 170 arespectively. The total number of electrode extensions is designated asK.

Although FIG. 1C shows rectangular interdigital electrodes 150/170 withparallel extensions 150 a/170 a of same geometry and spacing G, otherelectrode geometries are also contemplated. Design considerationsinclude the gap between electrodes, the length of the electrode, and thenumber, if any, and shape of electrode extensions. The gap can be usedto control coupling between the input and output electrodes. Longerelectrodes can also increase coupling. The number of extensions K can beused to control the bandwidth and/or to increase coupling whileconserving the area taken up by the electrodes. In some embodiments, theelectrodes are composed of rectangular strips, with two or moreextensions (e.g., K≥2). For example, each extension can be a rectangularstrip. In some embodiments, the electrodes are concentric circles orspirals having a common axis.

Piezo layer 110 can be formed from various piezoelectric materials.Exemplary materials include ZnO, AlN, CdS, PZT, LiNbO₃, LiTaO₃, quartz,KNN, BST, GaN, Sc alloyed AlN, or the aforementioned materials doped oralloyed with an additional element. Doping can be used to improve ortailor electromechanical properties of piezo layer 110. As detailedfurther below, piezo layer thickness d is selected such thatthickness-extensional modes near the frequencies of the desiredbandwidth of the LBAW filter are produced in the piezo layer. In someembodiments, piezo layer thickness d is 20% to 50% of λ_(z), or 30% to45% of A_(z), where λ_(z) is the wavelength of the piezoelectricvibration in the thickness direction. In some embodiments, d is 1500 nmto 2500 nm, or 1800 to 2200 nm.

Thin film IDT 102 can be composed of various materials. In someembodiments, IDT electrodes 150 and 170 are metal. For example, theelectrode material can include Al, Mo, Pt, Cu, Au, Ag, Ti, W, Ir, Ru, ormultilayers of metals and/or metals doped with additional materials,e.g. AlSi, AlSiCu, polysilicon, etc. Doping can be used to improve ortailor IDT electric or mechanical properties.

Although FIG. 1A shows a single common counter electrode 120, filter 100can include separate electrodes for the input and output resonators.Various materials are suitable for the counter electrode(s) (e.g.,electrode 120). For example, the electrodes can include a metal, such asAl, Mo, Pt, Cu, Au, Ag, Ti, W, Ir, Ru, or multilayers of metals and/ormetals doped with additional materials, e.g. AlSi, AlSiCu etc. Dopingcan be used to improve or tailor IDT electric or mechanical properties.For example, the electrodes can be Ti+Mo, Ti+W, AlN+Mo, or Al+W. Theelectrodes can be multilayered. The electrodes can have a special thinseed layer deposited below the electrode.

Reflecting structure 130 can be composed of alternating layers ofdifferent materials. For example, reflecting structure 130 can includealternating layers of two of: tungsten (W), SiO₂, silicon (Si), andcarbon (C). For example, layers of high acoustic impedance include W,Mo, Ir, Al₂O₃, diamond, Pt, AlN, Si₃N₄. Layers of low acoustic impedancecan include SiO₂, glass, Al, Ti, C, polymers, or porous materials. Layerof Si provides an intermediate acoustic impedance. Various materials aresuitable for the substrate 140, such as Si or SiO₂ or glass, sapphire,quartz. Substrate 140 materials can have high electrical resistivity.The substrate can have a thickness appropriate for RF applications, suchas integration into mobile phone platforms. For example, the substratecan have a thickness less than 500 microns, or less than 200 microns.For example, Si wafers can be purchased with a thickness of 675 μm andthinned down to a achieve desired device thickness, e.g., for mobileplatforms.

Modeling of the acoustic response of LBAW 100 can provide guidance onhow to tune the design parameters for individual elements of thestructure to achieve desired bandpass properties. For example, LBAW 100can be designed to have resonance modes at specific frequencies. Ingeneral, the geometry of various LBAW 100 components can be selected toachieve various acoustic properties. LBAW 100 properties can depend onthe combination of these geometries, which may not be independent of oneanother.

In piezoelectric layer 110, different bulk acoustic vibration modes canarise at different excitation frequencies f of input voltage (e.g., atport 160). Acoustic vibrations in piezo layer 110 can propagatelaterally as Lamb waves (or plate waves), wherein particle motion liesin the plane that contains the direction of wave propagation and theplate normal (e.g., the z-axis in FIG. 1A). Two such modes are shown inFIGS. 2A-2B. Referring to FIG. 2A, a thickness-extensional (TE orlongitudinal) bulk mode 200 has particle displacement 210 dominantlyperpendicular to the propagation direction (in the z-direction).Referring to FIG. 2B, a second order thickness-shear (TS2) bulk mode 220has particle displacement 230 dominantly parallel to the propagationdirection (in the y-direction). For both modes, the lowest frequency atwhich resonance in the thickness direction can arise is when thethickness d of piezo layer 110 is equal to an integer number of halfwavelengths λ_(z) (disregarding the thickness of electrodes 150/170); inother words, when

${d = \frac{N\; \lambda_{z}}{2}},$

where N is an integer that indicates the order of the resonance. For theTE1 mode,

$d = {\frac{\lambda_{z}}{2}.}$

As discussed further below, the width W of the electrodes and the gap Gbetween electrodes can be designed such that TE1 mode standing waveswith certain lateral wavelengths λ_(∥) are formed that can couplethrough their evanescent tails across gap G to create two mechanicalresonant modes.

Acoustic properties of an LBAW resonator 100 can be described withdispersion curves. Referring to FIG. 3, an example dispersion curve foran LBAW 100 shows the lateral wave number k_(∥) of the vibration, where

${k_{\parallel} = \frac{2\pi}{\lambda_{\parallel}}},$

as a function of frequency f. The first-order longitudinal (thicknessextensional, TE1) vibration mode, in which the combined thickness of thepiezoelectric layer d and the thickness of electrode(s) 150 or 170contains approximately half a wavelength of the bulk vibration, λ_(z)/2,and the second-order thickness shear (TS2) mode, in which the bulkvibration is dominantly perpendicular to the thickness direction (z-axisin FIG. 2B) and approximately one acoustic wavelength λ_(z) is containedin the combined piezoelectric layer thickness d and the thickness ofelectrode(s) 150 or 170, are denoted in the figure. The TE1 mode is thedarker portion of each dispersion curve, and TS2 mode is the lighterregion of each dispersion curve. The top curve (“no electrode”)represents the dispersion properties of the piezoelectric layer underthe gap 190. The bottom curve (“electrode”) represents the dispersionproperties of the piezoelectric layer under electrodes 150/170, alsoknown as the active region. More specifically, where the “electrode”curve intersects k=0, the TE1 mode has approximately λ_(z)/2 containedin the combined thickness of the electrodes 150 or 170 and thepiezoelectric layer. This is approximate because the wave can extendinto the Bragg reflector. “No Electrode” curve intersection with k=0lines shows the modes where approximately λ_(z)/2 is contained in thecombined thickness of the bottom electrode only and the piezolayer. Thistype of dispersion, in which the TE1 mode has increasing k_(∥) withincreasing frequency f, is called Type 1. The difference in intersectk∥=0 frequencies between electrode and non-electrode areas determinedthe hard limits for the achievable bandwidth of the filter. The gapwidth G, electrode width W, and number of extensions K can be used tovary the coupling strength within the limits set by the dispersiondifference.

In some embodiments, LBAW 100 can be designed to produce Type 1dispersion. For example, piezo layer 110 materials can be selected inwhich Type 1 dispersion can occur. For example, ZnO can be used. Inanother example, appropriate design of acoustic Bragg reflector 130 canhelp achieve Type 1 dispersion. For example, using Aluminum nitride(“AIN”) for piezo layer 110 can typically produce a Type 2 dispersion,where TE1 mode behaves non-monotonically having initially decreasingk_(∥) with increasing frequency f, and then increasing k_(∥) withincreasing frequency f, (roughly similar to what is described in thedispersion curves of in FIG. 3 but with TE1 and TS2 interchanged).However, in some embodiments, with an appropriate design of thereflecting structure 130 (e.g., acoustic Bragg reflectors), the LBAW 100can use AIN in piezo layer 100 and still achieve a Type 1 dispersion.See for example Fattinger et al. “Optimization of acoustic dispersionfor high performance thin film BAW resonators,” Proc. IEEE InternationalUltrasonics Symposium, 2005, pp. 1175-1178.

In FIG. 3, positive values of kH denote real wave numbers (propagatingwaves) and negative k∥ values correspond to imaginary wave numbers(evanescent waves). For a resonance to arise, the acoustic energy mustbe trapped inside the LBAW resonator structure. In the thickness(z-axis) direction, isolation from the substrate (using reflectingstructure 130) can be used for energy trapping. In the lateraldirection, energy trapping can occur when an evanescent wave formsoutside the electrode region (e.g., on the “no electrode” curve). To getresonant coupling between the two resonators (e.g., electrodes 150/170and 120) of an LBAW, standing waves of a TE1 mode form in the activeregions of the piezo layer (under the electrodes), and evanescent wavesform in the “no electrode” region. In other words, k∥ is positive forthe TE1 “electrode” curve and negative for the TE1 “no electrode” curve.According to FIG. 3, this occurs in the labeled “trapping range”frequency range. Energy trapping can be easier to realize in Type Idispersion. Without wishing to be bound by theory, with monotonicallyincreasing dispersion curves as the thick TE1 lines in FIG. 3, for the“Electrode”, at a single frequency in the trapping range there is eithera single imaginary wave number available or above the trapping range asingle real wave number. The former means that the TE1 does notpropagate outside the electrode and the latter that the TE1 can coupleto a propagating wave outside the electrode and thus “leak”. The Type 2dispersion can be described by similar curves but with the TE1 and TS2curves interchanged. The fact that the curve in Type 2 is non-monotonicmeans that at a given frequency there may be several real wavenumbers.Having several wavenumbers for a frequency means propagating waves areavailable outside the electrode, which can cause a “leak”.

FIGS. 4A-4B illustrate the relationship between standing wave resonancemodes and the LBAW bandgap. Referring to FIG. 4A, a portion of LBAW 100includes two adjacent electrodes 401 and 402 with width W (e.g.,corresponding to extensions 150 a and 170 a of respective electrodes 150and 170 of FIG. 1A). The bandpass frequency response of LBAW 100 isformed by two or more laterally standing resonance modes, 410 and 420,arising in the structure. Lateral standing wave resonance can arise whenplate waves are reflected from edges of the adjacent electrodes 401 and402. In the even mode resonance 410, the piezo layer under bothelectrodes 150 and 170 vibrates in-phase, whereas in the odd moderesonance 420, the phases are opposite. The even lateral standing waveresonance can arise when the total width of the structure is roughlyequal to half of the lateral wavelength λ_(∥) of the mode:

$\frac{\lambda_{even}}{2} = {\frac{\lambda_{\parallel}}{2} \approx {{2 \cdot W} + {G.}}}$

In the limit of infinitely small gap width G, λ_(even) approaches thetotal width from below. As shown in FIG. 4A, λ_(even) gets smaller whenG gets larger and gets larger when G gets larger. In case of small gap(e.g., zero-gap) λ_(even) gets close to 4W and in case of a large gapλ_(even) gets close to 2W. The odd lateral standing wave resonance canarise when the width of the electrode is roughly equal to half of thelateral wavelength λ_(|) of the mode:

$\frac{\lambda_{odd}}{2} = {\frac{\lambda_{\parallel}}{2} \approx {W.}}$

Referring to FIG. 4B, the even 410 and odd 420 modes are shown astransmission peaks as a function of input frequency f for an LBAW withType 1 dispersion. For Type 1 dispersion, the even mode 410 has a longerwavelength and is lower in frequency than the shorter-wavelength oddmode 420. The frequency difference 430 between the modes determines theachievable bandwidth of LBAW filter 100, and depends on the acousticproperties of the structure and on the dimensions of IDT resonator 102.Acoustic coupling strength can be defined in terms of the (resonance)frequency difference between even (symmetric) and odd (antisymmetric)resonances:

$\frac{f_{asymm} - f_{symm}}{f_{0}},$

where f_(symm) and f_(asymm) are the symmetric and antisymmetriceigenfrequencies, respectively, and f₀=(f_(symm)+f_(asymm))/2 is thecenter frequency between the two modes.

In some embodiments, increasing the number of extensions (e.g., 150 aand 170 a) in each electrode (e.g., 150 and 170) can increase thefrequency difference between the even and odd mode in the LBAW, and thusincrease the bandwidth. This effect can result from the fact that thelateral wavelength of the odd mode can depend on the periodicity of theelectrode structure (e.g., width W), while the even mode can depend onthe entire width of the structure (e.g., adding up all widths W and gapsG). For example, if the total number of electrode extensions K, theelectrode width is W, and the gap width is G, the wavelength λ_(∥) ofthe lateral acoustic wave at the even mode resonance frequencyapproaches or is slightly shorter than:

$\frac{\lambda_{even}}{2} \approx {{K \cdot W} + {K \cdot {G.}}}$

The odd lateral standing wave resonance in this structure, however,approaches or is slightly larger than:

$\frac{\lambda_{odd}}{2} \approx {W.}$

Additionally, or alternatively, in some embodiments, the total width ofthe structure K·W+K·G can be such that the highest-order mode trapped inthe structure is the desired odd mode resonance. For example, K can be31, W can be 3 μm, and G can be 2 μm.

In some embodiments, the number of electrode extensions K is between 2and 200, or between 10 and 60. In some embodiments, the length L ofelectrode extensions can be between 50 μm and 2000 μm, or between 70 μmand 500 μm.

In some embodiments, the gap G is selected to allow coupling of theevanescent tails of standing waves formed under electrodes 150 and 170.For example, the gap G between electrode extensions can be 0.1 μm and 10μm, or between 2 μm and 5 μm.

In some embodiments, the topology of the electrodes 150 and 170 can bedesigned such that the gap width G provides good enough coupling betweenelectrode extensions to create a single even mode 410 across the entirewidth of the structure. For example, the gap width G can be 2%-300%, or10%-100% of the evanescent acoustic wave's decay length, i.e. the lengthat which amplitude A=A₀·e⁻¹ of the original amplitude A₀, in the gap atthe desired even resonance mode. The gap width G can be optimized.Decreasing the gap to a too small width (1) can eventually pull the evenand odd modes too far from each other creating a dip in the passband,(2) can lead to reduced coupling coefficient for the odd mode, or (3)can increase capacitive feedthrough from finger to finger causing poorout of band attenuation.

In some embodiments, the gap width G can be defined with respect topiezo layer thickness d. For example, G can be designed to be 10% to300% of d, or 25% to 150% of d.

In some embodiments, the width of electrode extensions W can be between0.1 μm and 30 μm or between 2 μm and 5 μm. In some embodiments, W can bedesigned such that the wavelength λ_(∥) of the lateral acoustic wave atthe desired odd mode resonance frequency λ_(odd) is obtained.

In some embodiments, the electrode width W is designed such thatmultiple half-wavelengths cannot fit within the electrode width. Forexample, W can be designed to be smaller than the lateral acousticwave's wavelength λ_(∥) at the desired odd resonance mode, e.g., whereλ_(∥)=λ_(odd).

In some embodiments, the thicknesses of various LBAW 100 components canbe selected to achieve various acoustic properties and may beinterdependent. For example, the piezo layer 110 thickness d (minimumand maximum value) can first be determined with respect to the acousticwavelength in the piezo material (λ) at the operation frequency f. Insome embodiments, thicknesses (min and max) of the other LBAW 100 layerscan be selected based on the choice of piezo thickness d. For example,the combined thickness of the electrodes (including the counterelectrode 120) and the piezoelectric layer can be selected to beapproximately half a wavelength of the mode that is being used, forexample longitudinal bulk wave for the thickness extensional mode.Fundamental modes with N=1 (the first mode, i.e., first harmonic) canallow for greater coupling, but N>1 modes are also possible. Forexample, the thickness of electrodes 150 and 170, bottom electrode 120,and reflecting structure 130 can be defined as a percentage of piezolayer thickness d. In some embodiments, once all thicknesses areselected, the geometry of the electrode extensions 150 a and 170 a, suchas number K, width W, and length L, can be tuned to match the LBAW 100electrical impedance to the system impedance. Without wishing to bebound by theory, impedance matching can help avoid losses andreflections in the system.

In some embodiments, thickness of electrodes 150 and 170 is between 1%to 30% of d, or 5% to 25% of d, or 3% to 15% of d.

In some embodiments, the thickness of bottom electrode 120 is between 5%to 50% of d, or 10% to 30% of d, or 10% to 20% of d.

In some embodiments, where the reflecting structure 130 is a Braggreflector, the alternative layers of the reflector can be designed suchthat the required reflectivity of passband wavelengths is obtained. Forexample, the thickness of each layer can be equal to or less or morethan one quarter of the acoustic wavelength A in the thickness directionto reflect the odd and even TE1 resonance modes. In some embodiments, asingle layer in the Bragg reflector can be 15% to 80% of d, or 20% to70% of d.

The mass loading of the IDT 102, determined by the thickness andmaterial of electrodes 150 and 170, can be designed such that thefrequency difference between the k_(∥)=0 frequency of the electroderegion's TE1 mode and the outside electrode region's TS2 mode is small.Without wishing to be bound by any particular theory, when the frequencydifference between outside region's TS2 mode and electrode region's TE1mode is small, the trapping range is large. More particularly, thek_(∥)=0 frequency of the outside region's TS2 mode can be 95%-99% of theelectrode region's TE1 cutoff frequency. The frequency differencebetween the outside region's TS2 and outside region's TE1 modes' k_(∥)=0frequencies is designed to be large, e.g. 5%-15%, for example 6.5%-7.5%,of the electrode region's TE1 mode cutoff frequency.

According to certain embodiments of the present invention, the k=0frequency of the outside region's TS2 mode is greater than, or equal to98%, or between 98% and 99.5%, or is 98.9% of the electrode region's TE1cutoff frequency. Similarly, the frequency distance expressed as thefrequency difference between electrode region TE1 and outside region TS2k_(∥)=0 frequencies:

$\frac{{{electrode}\mspace{14mu} {TE}\; 1} - {{outside}\mspace{14mu} {TS}\; 2}}{{outside}\mspace{14mu} {TS}\; 2}$

should be small, for example on the order of 1%. As an example, saidfrequency distance can be between 0.2% and 2.1%, or between 0.5% and1.8%, or between 0.8% and 1.5%, or for example, 1.1%.

FIG. 5 shows a curve of insertion loss IL versus frequency f for anexemplary LBAW 100. The curve shows two passbands with peak 510corresponding to TE1 waves and peak 520 corresponding to TS2 waves. Asdiscussed above, the width of each passband is determined by thefrequency difference of the even and odd modes for the respective typeof wave. Here, the TS2 modes correspond to sideband 520 a (also referredto herein as “TS2 passband”), and the TE1 modes correspond to passband510 a (also referred to herein as “TE1 passband”). In some embodiments,LBAW 100 is designed to suppress peak 520 corresponding to TS2 modes,while maintaining the properties of peak 510 corresponding to TE1 modes.Without wishing to be bound by any particular theory, TE1 mode operationcan be selected because piezo thin film materials have electromechanicalcoupling that is stronger in the thickness direction. In other words,TE1 longitudinal mode vibrations couple more efficiently to theelectrical excitation over the thickness of piezo layer 110.

In some embodiments, LBAW 100 can be designed to have a passband for TE1modes between 0.5 and 10 GHz, or between 1 and 4 GHz. In some examples,TE1 passband is between 1.8 and 3.7 GHz. The limits of the passband canincorporate design considerations. For example, the dimensions of thedevice can grow very large or very small. Too large dimensions may taketoo much space and cause inefficiencies. Too small dimensions candeteriorate performance due to thin and narrow electrodes leading toresistance and losses. In some embodiments, LBAW 100 can be designed tohave a TE1 passband width 510 a of 0.5-15% relative to center frequency,e.g., 10% relative to center frequency, or 5%, or 2%, or 1%. In someembodiments, the insertion loss at the passband is −7 dB to −0.5 dB or−5 dB to −1.5 dB.

Implementations of the present disclosure provides techniques tosuppress LBAW sidebands created by TS2 mode. The implementationssuppress the sidebands by cascading multiple LBAWs. The cascaded LBAWfilters can be designed to have different TS2 resonance frequencies sothat insertion loss of one filter suppresses sideband of the otherfilter.

FIGS. 6A-6D illustrate block diagrams of example filter devices 600 thatincludes two LBAW filters connected in series. The embodiment 600includes a first LBAW 100 a connected to a second LBAW 100 b. Each ofthe first and the second LBAWs 100 a and 100 b is structurally similarto LBAW 100 of FIG. 1A or 1B. For example, the first LBAW 100 a includesextensions 150 a, 170 a, and electrodes 150 and 170. The electrodes 150and 170 of the first LBAW 100 a extend substantially in parallel and areseparated from each other by a first gap. Similarly, the second LBAW 100b includes extensions 650 a, 670 a, and electrodes 650 and 670. Theelectrodes 650 and 670 of the second LBAW 100 b extend substantially inparallel and are separated from each other by a second gap.

Electrode 150 is an input electrode and electrode 170 is an outputelectrode of the first LBAW 100 a, and electrode 670 is an inputelectrode, and electrode 650 is an output electrode of the second LBAW100 b. The first and the second LBAWs 100 a and 100 b are configuredsuch that application of a radio frequency voltage between an electrode(e.g., electrode 150) of the first LBAW 100 a and the first counterelectrode 640 creates acoustic thickness-extensional (TE) and acousticthickness-shear (TS) resonance modes between the electrodes 150 and 170(e.g., in the first piezoelectric layer) of the first LBAW 100 a, andbetween the electrodes 650 and 670 (e.g., in the second piezoelectriclayer) of the second LBAW 100 b. Thus, the input electrode 150 of thefirst LBAW 100 a provides the input for the filter device 600, whereasthe output electrode 650 of the second LBAW 100 b provides the outputfor the filter device 600. Electrode 170 is electrically coupled withelectrode 670. The electrodes 170 and 670 (and connection 660 ifpresent) can form a common floating electrode 602 between the inputelectrode 150 of the first LBAW 100 a and the output electrode 650 ofthe second LBAW 100 b.

In some embodiments, electrode 170 is connected to electrode 670 througha conductive connector 660. FIGS. 6A and 6C illustrate examples ofconnection of electrodes 170 and 670 through the conductive connector660. As shown in FIG. 6A, the conductive connector 660 can be relativelyshort such that the first and second LBAWs 100 a, 100 b are basicallyadjacent, e.g., separated by less than the width of a LBAW device on thesubstrate, or less than a width of an electrode 170 or 760 (width can bemeasured along the direction parallel to the long axis of theextensions).

Alternatively, as shown in FIG. 6C, the conductive connector 660 can bean extended conductive body or conductive line, such that the first andsecond LBAWs 100 a, 100 b are separated by more than the width of a LBAWdevice. Although FIG. 6C illustrates the connector 660 as a straightline, this is not required. This permits effectively arbitrary placementof the LBAWs, which may be beneficial for layout of an integratedcircuit.

The conductive connector 660 can have up to the same length as theelectrodes 170, 670 (measured along the long axis of the electrodes 170,670, e.g., perpendicular to the long axis of the extensions 170 a, 670a). The conductive connector 660 can have a length (again, measuredalong the long axis of the electrodes 170, 670) that is greater than,less than, or the same as the width W of the extension 170 a, 670 a. Asshown in FIGS. 6A and 6C, the conductive connector 660 can have a lengththat is greater than the width of the extensions 170 a, 670 a.Electrodes 170, 670, and the conductive connector 660 together form acommon electrode 602.

In some embodiments, a filter device may have no conductive connector660, as depicted in FIG. 6B. In this case, the common electrode 602depicted in FIG. 6B functions as both electrode 170 of the first LBAW100 a and electrode 670 of the second LBAW 100 b. As noted earlier, thecommon electrode 602 can form a floating electrode between the inputelectrode 150 of the first LBAW 100 a and the output electrode 650 ofthe second LBAW 100 b.

In some embodiments, the common electrode 602 consists of individualfingers; the fingers do not extend from a conductive base and need notbe electrically connected. FIG. 6D illustrates an example filter devicewith multiple common electrodes between its two LBAWs 100 a and 100 b.The individual fingers 650 can be conductively separated from eachother. The fingers can be interdigitated with the extensions (150 a, 650a) of the electrodes (150, 650) of the two LBAWs 100 a and 100 b.

The electrodes 150, 650, and the common electrode 602 (that provide thebase of the comb structures) are spaced apart. The electrodes 170, 670,602 can be rectangular bodies, and can extend substantially in parallel(i.e., their long axes are substantially parallel). Alternatively,electrodes 150 and 170 can be arranged to form an angle greater thanzero with respect to electrodes 650 and 670. Two opposing electrodes (asparts of one LBAW or as parts of the common electrode 602) can havesimilar or different shapes and/or dimensions.

Each of the electrodes 150, 170, 650, 670 includes two or moreextensions 150 a, 170 a, 650 a, 670 a, respectively. The extensionsextend from and are spaced along the length of the respective electrode.The extensions 150 a, 170 a of the first LBAW 100 a may be substantiallyin parallel (e.g., form an angle less than 5 degrees) to the extensions650 a, 670 a of the second LBAW 100 b. A first set (or all) ofextensions of the first LBAW 100 a may be arranged so that the first setof extensions form an angle greater than zero (e.g., greater than 5degrees) with respect to a second set (or all) of extensions of thesecond LBAW 100 b (i.e., be not in parallel).

In each of the first and the second LBAWs 100 a, 100 b, each of theextensions can be part of a comb-shape structure extending from therespective electrode. The comb structure that extends from a firstelectrode (e.g., electrode 150) can be integrated with the combstructure that extends from a second electrode (e.g., electrode 170)that is opposite the first electrode. Each comb structure can includetwo or more extensions. Extensions of a comb structure can be inparallel with each other. Extensions of two integrated comb structuresare separated by a gap. Extensions of a first comb structures can be inparallel with the extensions of a second comb structure that isintegrated with the first comb structure. For examples, extensions 670 acan be in parallel with extensions 650 a of the second LBAW 100 b.

The two LBAWs 100 a, 100 b, can have the same or different number ofextensions (e.g., K can be the same for both LBAWs). For example, asshown in FIGS. 6A-6C, the comb structures associated with electrodes 170and 670 have the same number of extensions, and the comb structuresassociated with electrodes 150 and 650 in FIGS. 6A-6C have the samenumber of extensions, so that the total number of extensions is the samefor both LBAWs 100 a, 100 b.

Two interdigitated comb structures within an LBAW can have the same ordifferent number of extensions. For example, as shown in FIG. 6A, thecomb structure associated with electrode 150 has a different number ofextensions from the comb structure associated with the electrode 170.

Rather than two comb structures, a filter device may include a firstelectrode that provides a comb structure with multiple extensions, and asecond electrode that includes a single bar-shaped conductive body(e.g., a single extension) extending between the extensions of the comb.The comb structure is integrated by the bar extension.

The electrodes 150, 170, 650, 670 can be composed of the same ordifferent materials. The extensions 150 a, 170 a, 650 a, 670 a can becomposed of the same or different materials. One or more electrodes andone or more extensions can be composed of the same or differentmaterials. In some examples, the extensions and the electrodes arecomposed of one or more metals, such as aluminum (Al).

FIG. 6E illustrates a cross sectional view of the filter device 600illustrated in FIG. 6A. The implementations of FIGS. 6B-6D would have asimilar cross sectional structure. For simplification, only portions ofthe filter device 600 are depicted in FIG. 6E.

The first LBAW 100 a includes a first acoustic reflector 608, a firstintermediary layer 606, e.g., a first dielectric layer, on top of (andthat in some implementations forms part of) the first acoustic reflector608, a first counter electrode 604 on top of the first intermediarylayer 606, a first piezoelectric layer 602 on top of the first counterelectrode 604, and one or more first conductive layers 610 on top of thefirst piezo electric layer 602. The one or more first conductive layers610 form the electrodes 150, 170, and the extensions 150 a, 170 a.

Similarly, the second LBAW 100 b includes a second acoustic reflector618, second intermediary layer 616, e.g., a second dielectric layer, ontop of (and that in some implementations forms part of) the secondacoustic reflector 618, a second counter electrode 614 on top of thesecond intermediary layer 616, a second piezoelectric layer 612 on topof the second counter electrode 614, and one or more second conductivelayers 620 on top of the second piezoelectric layer 612. The one or moresecond conductive layers 620 form the electrodes 650, 670, and theextensions 650 a and 670 a. In some implementations, the first and thesecond intermediary layers 606 and 616 can be conductive layers. Suchconductive layers can be made of aluminum.

The first and/or the second intermediary layers can be two regions of anintermediary layer. The intermediary layer can be the topmost layer ofthe acoustic reflector 608 or can be positioned on top of the topmostlayer of the acoustic reflector. The two regions that include the firstand the second intermediary layers can have different thicknesses. Theacoustic reflector 608 can have one or more layers. The topmost layer ofthe acoustic reflector 608 is a layer that is closest to the counterelectrode from among the layers of the acoustic reflector 608.

In some embodiments, the first and/or the second reflector is a Braggmirror. The first and the second reflectors 608 and 618 can be parts ofa common reflector shared between the first and the second LBAWs 100 aand 100 b. The first and the second piezoelectric layers 602 and 612 canbe parts of a common piezoelectric layer shared between the first andthe second LBAWs 100 a and 100 b.

Cascaded LBAWs can be utilized to suppress spurious sidebands created byTS modes. For example, embodiment 600 may have a sideband that is aresult of sidebands of the first and the second LBAWs 100 a and 100 b.The first LBAW 100 a and/or the second LBAW 100 b can be utilized tosuppress the sideband of embodiment 600 to have a lower signaltransmission compared to the individual ones of LBAW 100 a and 100 b.

Resonance frequencies corresponding to TE1 mode (f_(TE1)) and Resonancefrequency corresponding to TS2 mode (f_(TS2)) of an LBAW depend ongeometry and characteristics of the layers of the LBAW. The resonancefrequency ratio f_(TE1)/f_(TS2) of an LBAW is a function of the Poissonratio of the materials used in the LBAW. The Poisson ratio of amultilayer structure with a stack of two materials A and B is a functionof t_(A)/t_(B), where t_(A) is thickness of a layer composed of materialA, and t_(B) is thickness of a layer composed of material B.Accordingly, resonance frequency ratio f_(TE1)/f_(TS2) of an LBAWdepends on thickness of the LBAW's layers with respect to each other andcan be adjusted by changing thickness of one or more layers of the LBAW.For example, resonance frequency ratio of an LBAW can be adjusted bychanging thickness of dielectric layer, piezo layer, etc. of the LBAW.Thus, the two LBAWs 100 a, 100 b can have different resonancefrequencies by providing them with dielectric layers of differentthicknesses.

The first and the second intermediary layer 606, 616 can be composed ofthe same material, e.g., silicon oxide (SiO2), silicon nitride (SiN),etc. However, as described further below, the first and the secondintermediary layer 606, 616 can have different thicknesses.

In the example embodiments depicted in FIGS. 6A-6E, other than asdescribed below (e.g., for the intermediary layers 606, 616), the secondLBAW 100 b can have layers identical to the layers of the first LBAW 100a (e.g., layers with identical thicknesses and materials).

To tune TS2 resonance frequency of the first LBAW 100 a, the secondintermediary layer 616 can be thicker (or thinner) than the firstintermediary layer 606. In some embodiments, thickening the secondintermediary layer 616 causes resonance frequency ratio f_(TE1)/f_(TS2)of the second LBAW 100 b to increase while the absolute values f_(TE1)and f_(TS2) are both decreased. The thickness change of the intermediarylayer also causes a shift in TS2 resonance frequency which is largerthan the change in TE1 resonance frequency. For example, thickening thesecond intermediary layer lowers the TS2 resonance frequency, andthinning the second intermediary layer increases the TS2 resonancefrequency of the second LBAW 100 b.

In the present disclosure, the two intermediary layers are fabricated sothat one intermediary layer is thicker than the other intermediarylayer. For example, the intermediary dielectric layer 616 can be thickerthan the first intermediary layer 606 (as shown in FIG. 6E), or viceversa. For example, one intermediary layer, e.g., one dielectric layer,can be fabricated as the thicker layer by depositing more material onthe acoustic reflector compared to the other layer, and/or by thinningdown the other intermediary layer (e.g., by etching) more than the oneintermediary layer.

FIG. 7A depicts transmission curves of two individual example LBAWs. Thetwo individual LBAWs can be the first LBAW 100 a and the second LBAW 100b with a second dielectric layer 616 thicker than the first dielectriclayer 606. As illustrated, FIG. 7A includes a first transmission curve710 associated with the first LBAW 100 a, and a second transmissioncurve 720 associated with the second LBAW 100 b. The first transmissioncurve 710 includes a first sideband with a first peak 710 a. The secondtransmission curve 720 includes a second sideband with a second peak 720a. Due to the second dielectric layer 616 being thicker than the firstdielectric layer 606, the second sideband has shifted compared to thefirst sideband. Accordingly, the second peak 720 a is at a lowerfrequency compared to the first peak 710 a. Because of the difference inthe sidebands of the two LBAWs 100 a and 100 b, when the two LBAWs arecascaded (e.g., as depicted in filter device 600), the overall sidebandof the cascaded embodiment is suppressed and has a lower sideband peakcompared to each individual ones of LBAWs 100 a and 100 b.

FIG. 7B depicts dispersion curves of the two individual example LBAWs ofFIG. 7A. Dispersion curve 710 b is associated with the first LBAW 100 a,and dispersion curve 720 b is associated with the second LBAW 100 b witha thicker dielectric layer (e.g., thicker second dielectric layer 616).As explained with reference to FIG. 3, the upper portions of the curvesare related to TE1 modes and the lower portions are related to TS2 modesof the two LBAWs. A thicker second dielectric layer 616 increases thefrequency ratio f_(TE1)/f_(TS2) of the second LBAW 100 b, causing anincrease in the distance between TE1 and TS2 resonance frequencies ofthe second LBAW 100 b as compared to resonance frequency differences inthe first LBAW 100 a. With substantially similar TE1 passbands (e.g.,close frequency cutoffs), the increase in the distance between TE1 andTE2 resonance frequencies cause a suppression in signal transmission inthe sidebands, while keeping the passband almost intact.

FIG. 7C depicts transmission curve of an example filter device withcascaded LBAWs. The cascaded LBAWs can be the first LBAW 100 a and thesecond LBAW 100 b with a second dielectric layer 616 thicker than thefirst dielectric layer 606. Similar to FIG. 7A, FIG. 7C includes a firsttransmission curve 710 associated with the first LBAW 100 a as measuredindividually, and a second transmission curve 720 associated with thesecond LBAW 100 b as measured individually. A third transmission curve730 represents transmission of the filter device 600, where the firstand the second LBAWs 100 a and 100 b are cascaded.

As illustrated, the sideband 732 of the cascaded LBAWs is suppressedcompared to the passband 740. In particular, the sideband 732 can havemore than 30 dB loss, whereas the passband is less than 10 dB loss.Similarly, the sideband 732 of the cascaded LBAWs is suppressed comparedto each of the sidebands 712 and 722 associated with the individualLBAWs; the sideband peaks 730 a, 730 b of the cascaded LBAWs are smallerthan each peaks 710 a and 720 a of the individual LBAWs. The sidebandpeaks can be suppressed more than 20 dB, or even more than 30 dB bycascading LBAWs according to the present disclosure.

As explained above, the second intermediary layer 616 can be thicker orthinner than the first dielectric layer 606. In the example depicted inFIG. 6E, the second intermediary layer 616 is thicker than the firstintermediary layer 606. In some embodiments, the second intermediarylayer thickness is 1% to 10%, e.g., 2% to 7% greater than the firstintermediary layer thickness. In some embodiments, thickness of thethinner intermediary, e.g., dielectric, layer ranges between 900 nm and1100 nm, and thickness of the thicker intermediary, e.g., dielectric,layer ranges between 1000 nm and 1200 nm. In some examples, thickness ofthe thinner intermediary, e.g., dielectric, layer ranges between 1000 nmand 1050 nm, and thickness of the thicker intermediary, e.g.,dielectric, layer ranges between 1050 nm and 1100 nm.

In some embodiments, the thickness difference of the two intermediarylayers 606 and 616 causes a 1 to 10% difference between the resonancefrequencies of TS2 modes (i.e., frequencies of the sideband peaks 710 aand 720 a) of the two LBAWs 100 a and 100 b. A 2 to 7% thicknessdifference between the two intermediary layers can cause a 1 to 6%change in TS2 frequency. For example, a 2% thickness difference canprovide 1 to 2% change in TS2 frequency; a 5% thickness difference canprovide 2 to 4% change in TS2 frequency; a 7% thickness difference canprovide 4 to 6% change in TS2 frequency; and a 10% thickness differencecan provide 6 to 8% change in TS2 frequency. In some examples, theresulted difference between the resonance frequencies of the TS2 modesis at least 20 MHz, e.g., at least 50 MHz. In some examples, theresulted difference between the resonance frequencies of the TS2 modescan be between 50 to 200 MHz.

To make sure that the TE1 passband of the second LBAW 100 b is notsuppressed as a result of changing thickness of one or more layers, theTE1 resonance frequency of the second LBAW 100 b can be tuned to besubstantially close to (e.g., within 1% from, e.g., within 20 MHz, e.g.,within 10 MHz from) the TE1 resonance frequency of the first LBAW 100 a.Tuning the TE1 resonance frequency can be achieved by selectingdimensions of one or more of the second (or the first) conductive layers620, the counter electrode 614, and/or the piezoelectric layer 612. Forexample, the thickness of the extensions of the second (or the first)LBAW 100 b. For example, thickness of the extensions 650 a, 670 a,and/or the electrode 650, 670 can be adjusted by deposition or etchingprocesses. A thicker conductive layer provides a lower resonancefrequency, and a thinner conductive layer provides a higher resonancefrequency.

In the example embodiment depicted in FIG. 6E, extensions 650 a, 670 acan be thinner (e.g., be thinned) than the extensions 150 a, 170 a (orextensions of the two LBAWs 100 a and 100 b can be deposited withdifferent thicknesses). The width and length of the extensions aremeasured along axes parallel to the piezo layer (with the width beingthe dimension along which the extensions are interdigited), whereas thethickness of the extension is measured along the axis perpendicular tothe piezo layer. Therefore, by adjusting the thickness of multiplelayers of one or more cascaded LBAWs, spurious sidebands can besuppressed with a minimum (or little) effect on the passband. In someembodiments thickness of the extensions can be between 100 nm to 300 nm.The thicker extensions can be up to 20% thicker than the thinnerextensions. For example, the thicker extensions 150 a may have thicknessof 200 to 220 nm and the thinner extensions 650 a may have thickness of180 to 200 nm. The thicker extensions can be about 10% (e.g., less than13%) thicker than the thinner extensions. In some embodiments, thethickness of one or both electrodes 150, 170 can be adjusted to begreater than the thickness of one or both electrodes 650, 670, to tunethe TE1 modes of LBAWs 100 a and 100 b. For example, thickness of theelectrodes 650, 670 may be 5% to 20% smaller than the thickness ofelectrodes 150, 170.

Further, TE1 resonance frequencies can be tuned by selecting appropriateIDT geometries, such as width of the extensions, gap between theextensions, or the number of extensions of at least one of the LBAWs.For example, when the second dielectric layer 616 is thicker than thefirst dielectric layer 606, the extension width and/or gap between theextensions in the second LBAW 100 b can be selected to be lower than therespective parts in the first LBAW 100 a to tune TE1 passband of the twoLBAWs. The thinner extension widths can range between 1 to 7 μm. Thethicker extension width can range between 3 to 10 μm. In some examples,the thinner extension width ranges between 3 to 5 μm and the thickerextension width ranges between 4 to 6 μm. The thinner gap between theextensions can range between 0.5 to 2 μm. The thicker gap between theextensions can range between 1 μm to 3 μm. In some examples, the thinnergap between the extensions ranges between 0.5 μm and 1.75 μm, and thethicker gap between the extensions ranges between 1.5 μm to 3 μm.

In some embodiments, to adjust the TE1 passbands of the two LBAWs 100 aand 100 b, the number of extensions of the two LBAWs differ from eachother.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made without departingfrom the spirit and scope of disclosure. Accordingly, otherimplementations are within the scope of the following claims.

What is claimed is:
 1. An acoustic wave filter device comprising: anacoustic reflector comprising one or more layers; an intermediary layerproviding or on a topmost layer of the acoustic reflector, wherein theintermediary layer has a first region and a second region, the firstregion having a first layer thickness and the second region having asecond layer thickness different from the first layer thickness; a firstmultilayer stack on the acoustic reflector comprising a first counterelectrode on the first region of the intermediary layer, a firstpiezoelectric layer on the first counter electrode, and a first inputelectrode and a first output electrode on the first piezoelectric layer,the first input electrode and the first output electrode each having afirst electrode thickness and extending substantially in parallel andseparated by a first gap; and a second multilayer stack on the acousticreflector comprising a second counter electrode on the second region ofthe intermediary layer, a second piezoelectric layer on the secondcounter electrode, and a second input electrode and a second outputelectrode on the second piezoelectric layer, the second input electrodeand the second output electrode each having a second electrode thicknessand extending substantially in parallel and separated by a second gap,wherein the first output electrode is electrically connected to thesecond input electrode, and wherein the first multilayer stack andsecond multilayer stack are configured such that application of a radiofrequency voltage between the first input electrode and the firstcounter electrode creates acoustic thickness-extensional resonance modesin the piezoelectric layer between the first input and output electrodesand between the second input and output electrodes.
 2. The device ofclaim 1, wherein the second layer thickness is greater than the firstlayer thickness.
 3. The device of claim 1, wherein the radio frequencyvoltage between the first input electrode and the first counterelectrode layer creates second-order acoustic thickness-shear (TS2)resonance mode in the piezoelectric layer between the first input andoutput electrodes and between the second input and output electrodes,and the second layer thickness is different than the first layerthickness so that a first resonance frequency of TS2 resonance modecreated between the first input and output electrodes of the firstmultilayer stack differs from a second resonance frequency of TS2resonance mode created between the second input and output electrodes ofthe second multilayer stack.
 4. The device of claim 3, wherein the firstresonance frequency differs by 1% to 8% from the second resonancefrequency.
 5. The device of claim 3, wherein the first resonancefrequency differs from the second resonance frequency by at least 50MHz.
 6. The device of claim 1, wherein the second layer thickness is 1%to 10% greater than the first layer thickness.
 7. The device of claim 1,wherein the second electrode thickness is greater than the firstelectrode thickness.
 8. The device of claim 1, wherein the secondelectrode thickness is less than the first electrode thickness.
 9. Thedevice of claim 8, wherein the second electrode thickness is 1% to 10%thinner than the first electrode thickness.
 10. The device of claim 1,wherein each of the first input and output electrodes has multiple firstextensions, the multiple first extensions of the first input electrodeare interdigitated with the multiple first extensions of the firstoutput electrode, and each of the second input and output electrodes hasmultiple second extensions, the multiple second extensions of the secondinput electrode are interdigitated with the multiple second extensionsof the second output electrode.
 11. The device of claim 10, wherein eachof the first input and output electrodes is a comb structure having themultiple first extensions and wherein each of the second input andoutput electrodes is a comb structure having the multiple secondextensions.
 12. The device of claim 10, wherein the multiple firstextensions are thicker than the multiple second extensions.
 13. Thedevice of claim 12, wherein the second extension thickness is 5% to 20%smaller than the first extension thickness.
 14. The device of claim 12,wherein the second extension thickness is greater than the firstextension thickness so that a first resonance frequency of TE1 resonancemode created between the first input and output electrodes of the firstmultilayer stack is within 1% from a second resonance frequency of TE1resonance mode created between the second input and output electrodes ofthe second multilayer stack.
 15. The device of claim 12, wherein themultiple second extensions are substantially parallel to the multiplefirst extensions.
 16. The device of claim 12, wherein the multiplesecond extensions form an angle greater than zero with respect to themultiple first extensions.
 17. The device of claim 1, wherein theacoustic reflector is a Bragg mirror.
 18. The device of claim 1, whereinthe first output electrode and the second input electrode are parts of acommon electrode shared between the first multilayer stack and thesecond multilayer stack.
 19. The device of claim 1, wherein the firstoutput electrode is connected to the second input electrode through aconductive connector.
 20. The device of claim 19, wherein the conductiveconnector has a length extending parallel to a long axis of theextensions of at least one of the first and the second multilayer stacksthat is greater than a width of the extensions measured perpendicular tothe long axis of the extensions.
 21. The device of claim 1, wherein thefirst output electrode is substantially parallel to the second inputelectrode.
 22. An acoustic wave filter device comprising: an acousticreflector comprising one or more layers; an intermediary layer providingor on a topmost layer of the acoustic reflector, wherein theintermediary layer has a first region and a second region, the firstregion having a first layer thickness and the second region having asecond layer thickness different from the first layer thickness; a firstmultilayer stack on the acoustic reflector comprising a first counterelectrode on the first region of the intermediary layer, a firstpiezoelectric layer on the first counter electrode, and a first inputelectrode and a first output electrode on the first piezoelectric layer,the first input electrode and the first output electrode each having afirst electrode thickness and extending substantially in parallel andseparated by a first gap; and a second multilayer stack on the acousticreflector comprising a second counter electrode on the second region ofthe intermediary layer, a second piezoelectric layer on the secondcounter electrode, and a second input electrode and a second outputelectrode on the second piezoelectric layer, the second input electrodeand the second output electrode each having a second electrode thicknessand extending substantially in parallel and separated by a second gap,wherein the first output electrode is electrically connected to thesecond input electrode, and wherein the first multilayer stack andsecond multilayer stack are configured such that application of a radiofrequency voltage between the first input electrode and the firstcounter electrode layer creates acoustic modes in the piezoelectriclayer between the first input and output electrodes and between thesecond input and output electrodes.